The present invention relates to a method for modifying surface properties of shaped articles of an arcylic resin or, more particularly, to a method for reducing electrostatic charge on the surface of the shaped article and to enhance affinity to water of the surface of the shaped article of an acrylic resin by utilizing low temperature plasma.
As is well known, acrylic resins belong to one of the important classes of thermoplastic synthetic resins owing to their high anti-weathering resistance and high transparency and they are used widely for shaping various articles such as windshields of vehicles, dormer windows and skylights, lenses for taillights of automobiles, coverings for television screen, coverings for machines and instruments, sunglasses and the like.
Despite the excellent properties described above, shaped articles made of an acrylic resin have a defective point which limits the usefulness of the resin to a large extent. That is, the surface of an acrylic resin shaped article has poor affinity to water and becomes readily charged with static electricity which attracts dusts to be deposited thereon to a loss of the surface beautifulness. In addition, the surface is rather susceptible to scratches so that the transparency of the shaped article is rapidly lost after a relatively short period of use.
Of course, the problem of accumulation of electrostatic charge on the surface of a shaped article is not limited to the acrylic resins but also most of plastic materials suffer from this problem. Therefore, various attempts have been made to solve this problem. For example, the most conventional method is to coat the surface of the shaped article with an anti-static agent or to incorporate an anti-static agent into the resin composition before fabricating it into shaped articles.
These methods are, however, far from satisfactory. The former method of coating with an anti-static agent is defective in the poor durability of the anti-static effect in addition to the disadvantages of stickiness of the surface and blocking or sticking together of the shaped articles kept in contact with each other although the method is effective when an instant anti-static effect is desired.
The latter method of incorporating an anti-static agent into the resin composition is not so effective in increasing the affinity of the surface to water, although the surface resistivity can be decreased to some extent, so that the method is not always practical with its relatively low anti-static effect notwithstanding the durability of the effect. When the amount of incorporation of the anti-static agent is increased with an object to enhance the anti-static effect on the surface of the article, several drawbacks are unavoidable in the sticky touch of the surface, blooming or bleeding of the added anti-static agent and blocking of the articles kept in contact with each other along with decreased workability of the resin composition in molding as well as coloring of the surface, increased susceptibility to stain and decreased heat resistance.
Moreover, incorporation of an anti-static agent is basically to be avoided since even a small amount of an anti-static agent may greatly reduce the transparency of the acrylic resin shaped articles because the compatibility or affinity of an anti-static agent with the acrylic resin can never be perfect decreasing the beautifulness of the article to a great extent.
Recently, there has been proposed a method of a principle quite different from the use of an anti-static agent, in which the surface of a shaped article of an acrylic resin is exposed to low temperature plasma of a gas under a low pressure so that the affinity of the surface to water is improved and the accumulation of electrostatic charge on the surface can be reduced to some extent. The effectiveness of this method is, however, not always satisfactory.